1. Field of the Invention
The present invention relates a wireless communications terminal apparatus which is able to determine its present position, using radio frequency signals, and, more particularly, to such apparatus capable of accurate calculation of its position even when it is in the vicinity of a repeater.
2. Description of Related Art
Techniques for determining the position of a wireless communications terminal apparatus (mobile station), using signals transmitted from base stations in a mobile communications system have so far been proposed. For example, such a technique was proposed in JP-A No. 181242/1995 that the position of a mobile terminal is determined by using the positions of base stations and difference of propagation delay time of signals transmitted from the base stations to the terminal in a code division multiple access (CDMA) system.
Referring to FIG. 2, the above technique is explained. A mobile station MS receives signals from three base stations existing in its periphery performs correlation processing of the received signals, and determines timing of each signal reception. From the determined timing of each signal reception, delay time difference of signal reception timing from the base stations (in proportion to difference in distance of the mobile station from the base stations) is calculated. Using the thus calculated delay time difference, by evaluating the equations given in FIG. 2, the position of the mobile station can be obtained.
In a conventional mobile communications system in which the above-explained technique is applied, a repeater (RP) that receives signals from a particular base station BS1 and retransmits the signals is installed in addition to the base stations as is shown in FIG. 3. Such repeater is capable of extending the area within which cellular communications can be implemented with less cost. Thus, repeaters are widely used, especially for extending indoor service areas. However, the repeater RP receives signals transmitted by the particular base station BS1 and transmits the signals as is. Therefore, the repeater RP transmits the same signals as those that the base station BS1 transmits. The time of receiving a signal transmitted by the repeater is later than the time of receiving the signal directly transmitted from the base station BS1 due to signal processing on the repeater. A chart presented at the top of the page of FIG. 3 shows a delay profile of measured time of signal reception from the repeater in a wireless communications system in practical use. PN0 is a signal from the base station nearest to the mobile station and a correlation value representing intensive power of the signal is obtained. PN1 is a signal from the base station BS1; the signal being repeated by the repeater RP connected to the BS1. Timing when the mobile station MS would receive the signal directly from BS1 on the supposition that the signal is not repeated is marked with a dotted line in the chart.
When the mobile station comes near the repeater, it receives a signal of large delay repeated by the repeater. From the chart of the PN1 signal, it is seen that the mobile station receives the signal repeated by the repeater with delay equivalent to 12 km longer than the distance between the BS1 and the mobile station MS which would otherwise be calculated from the assumed time of reception of the signal directly transmitted from the BS1. Position calculation of the mobile station using the repeated signal results in a mobile station position with quite a large error.
Briefly, problems of the conventional method in which a wireless communications terminal (mobile station) determines its position are that we have had:
(1) No method of detecting a repeater; and
(2) No method of mitigating the influence of the repeater on the terminal position determination even if the repeater was detected by some means or other.